Leprosy reactions: Unraveling immunological mechanisms underlying tissue damage in leprosy patients

Abstract Leprosy is a chronic granulomatous infectious and disabling disease caused by two mycobacteria, Mycobacterium leprae and Mycobacterium lepromatosis. Acute inflammatory responses, known as leprosy reactions, are significant contributors to disabilities. Three types of leprosy reactions have been identified based on excessive cytokine release (e.g. type 1) or the accumulation of immune complexes in tissues inducing multiorgan damage (e.g. types 2 and 3). The type of leprosy reaction has implications on treatment and management strategies, yet are not well understood by health workers caring for leprosy patients. We attempt to describe the immunologic mechanisms behind the different leprosy reactions and the rationale for tailoring clinical treatment and management to the particular type of leprosy reaction based on the underlying immunologic situation.


Introduction
Leprosy is a chronic granulomatous infectious disease caused by two types of mycobacteria: Mycobacterium leprae and Mycobacterium lepromatosis (Cardona-Castro et al. 2022 ).Approximately 5% of the world's population is susceptible to infection, which, along with the immunological response to infection, can damage the eyes (e .g. lagophthalmos , keratitis , iritis , acute and c hr onic uv eitis, secondary glaucoma), cause neuropathy with functional and sensory loss, and produce deformities and ulcers to the hands and feet (Rathod et al. 2020, Deschênes 2023 ).Additionally, the infection and imm unologic r eaction to the bacterium can cause canonical neurological and dermatologic symptoms (e.g.erythematous macules , panniculitis , inflamed plaques , neuritis) (Nunzi et al. 2020 ).
On the other hand, leprosy presents a wide clinical spectrum.Indeterminate leprosy, the initial form of the disease, r epr esents a sta ge wher e symptoms ar e subtle and not clearly defined.Tuberculoid leprosy is characterized by localized skin lesions and nerve dama ge, while lepr omatous lepr osy is typified by widespread skin nodules and profound nerve inv olvement.Bor derline leprosy, positioned between these two extremes (tuberculoid and lepromatous), exhibits a combination of symptoms, featuring varying degrees of skin lesions and nerve damage (Nunzi et al. 2020 ).
Curr ent tr eatment to rid the patient of the bacterial infection involv es m ultidrug ther a py (MDT) comprising da psone, clofazimine, and rifampicin for six doses (paucibacillary forms) for PB and 12 doses (multibacillary forms) (Nunzi et al. 2020 ).While the MDT r esolv es the bacterial infection in most cases, it is a common misunderstanding that patients after MDT do not have further immunological concerns and thus need not be follo w ed clinically.The destruction of the bacterium by the MDT is not clean but instead leaves behind remnants of bacterial cells .T hese particles trigger a series of acute inflammatory episodes known as leprosy reactions (LRs), causing further short-term and long-term dama ge to patients, a ggr av ating dermatologic, neur ologic, and indeed multi-system symptoms discussed further below.T herefore , it is important to consider leprosy as more than an infectious disease; the long-term sequelae are often related to LRs from the perspective of an immunological disorder.
Importantl y, LRs ar e heter ogeneous in pathophysiology and clinical presentation and thus r equir e differ ent a ppr oac hes to clinical treatment and management (Wu and Boggild 2016 ).LRs cause variable dermatologic damage and pain, limiting the activities of patients, and also can be unpredictable both in frequency, onset, and response to therapy.They can also exacerbate the stigma and socio-economic losses that come with a leprosy diagnosis (Putri et al. 2022 ).T hus , it is important that treatment and response monitoring account for the underlying pathophysiology as well as the stigma and socioeconomic burden caused by LRs.Following the patient, often at more frequent intervals, is critical to ensure medical adherence and the prevention of relapse.

Gener al o vervie w of lepr osy reactions
LRs are commonly considered immune hypersensitivity responses that can manifest at any time before, during, or after completing lepr osy m ultidrug ther a py (MDT) (Pitta et al. 2022, Putri et al. 2022 ).To date, three distinct types of LR have been described: Type 1 leprosy reactions (T1R) trigger inflammation in the skin and peripheral nerves due to an excessive release of cytokines due to an increased Th1 and Th17 cellular immune response (i.e.type IV hypersensitivity) (Mitra 2018 ).Type 2 leprosy reactions (T2R) and type 3 leprosy reactions (T3R) are associated with m ulti-or gan and m ulti-tissue inflammatory r esponses, with T2R could involve type III hypersensiti vity (i.e.antigen-antibod y immune complex deposition) or an increase in Th17 cells, along with a decrease in T regulatory cells (Tregs) (Luo et al. 2021 ); T3R could be associated with type III hypersensitivity (Velarde-Félix et al. 2016 ).These systemic clinical manifestations are associated with the formation and accumulation of immune complexes within affected tissues, leading to disabilities and impacting the quality of life of the leprosy patients (Fonseca AB de et al. 2017 ).
For this reason, LRs are a critical issue in the context of leprosy, and we propose that understanding their immunological and cellular mechanisms will be crucial in the future to generate biomarkers that facilitate detection and promote the de v elopment of new strategies for managing these immune events .T his r e vie w anal yzes and describes the main imm unological mec hanisms associated with tissue damage induced by LRs.

Immune response in type 1 leprosy reactions (T1R)
T1R is c har acterized by an intensified host cellular imm une r esponse against either M. leprae or M. lepromatosis (Antunes et al. 2019 ).The heightened immune response not only leads to the elimination of the mycobacteria but also results in collateral damage to the infected tissues, notably the skin and peripheral nerves (Nery JA da et al. 2013 ).T1R can occur in a ppr oximatel y one-third of patients with borderline leprosy and is often called a "r e v ersal reaction" due to the hyperergic immune response mimicking a shift in the clinical-histological Ridley-Jopling classification as patients move from a borderline spectrum to a tuberculoid leprosy form (Naafs andvan Hees 2016 , Froes et al. 2022 ).
RR is a Type IV hypersensitivity reaction that induces a heightened activation of T lymphocytes (TL) featuring a Th1 and Th17 effector pattern (Serrano-Coll et al. 2018 ).Constistent with Th1 activity, TLs stimulate an increased release of IL-2, which creates a robust clonal expansion of CD8 TLs following a cytotoxic Tc1 pattern (Dewi et al. 2023 ).When CD8 TLs encounter residual mycobacterial antigens presented on major histocompatibility complex class I (MHC-I) molecules within infected cells in the skin and peripheral nerves, they initiate the release of perforins and gr anzymes, r esulting in neuritis and panniculitis in the subcutaneous adipose tissue of the skin (de Oliv eir a et al. 2013 , Abbas et al. 2015 ).Mor eov er, this inflammatory process in the skin and peripher al nerv es is further intensified by Th1 and Th17 CD4 TLs secreting IL-17, TNF-α, and IFN-γ (Nery JA da et al. 2013, Naafs and van Hees 2016, Fonseca AB de et al. 2017 ).These cytokines recruit neutrophils and macrophages, which adopt the classical M1 pr o-inflammatory phenotype, ther eby exacerbating tissue dama ge (Sc hmitz et al. 2019 ).In Fig. 1 , we describe canonical tissue damage mediated by T1R immunological mechanisms.
Inter estingl y , the T -cell dependency of Type 1 r eactions cr eated a concern that Type 1 reactions could be triggered by vaccinating leprosy patients against SARS-CoV-2 with the BNT162b2 and Cor onaVac v accines .T here ha v e been at least 7 cases of lepr osy patients receiving SARS-CoV-2 vaccines and then presenting with a Type 1 reaction.Ho w ever, in a UK retrospective cohort study, de Barros et al. (de Barros et al. 2023 ) could not to establish a relation between SARS-CoV-2 vaccination and an increase in the incidence of RR in leprosy patients or in tuberculoid leprosy cases in individuals with latent infections.Ho w e v er, further studies ar e needed to explore the association between SARS-CoV-2 vaccination and the de v elopment of LRs.
Finally, it is crucial to note that the heightened cellular immune response in T1R on peripheral nerves can quickly induce deformities and disabilities in the eyes , hands , and feet (Serrano-Coll et al. 2018 ), affecting the function and quality of life of these patients.The frequency of nerve damage during T1Rs can very high, ranging from between 43% and 73%, and underlines the importance of monitoring and treatment to prevent lifelong disability (Naafs and van Hees 2016 ).Nerve damage can also occur during T2R and can e v en a ppear alone in the context of no ongoing r eaction.

Immune response in type 2 leprosy reactions (T2R) or erythema nodosum leprosum
T2R is an immune event in leprosy patients that can be explained thr ough v arious imm une mec hanisms, including 1. Deposition of immune complexes at the tissue level, 2. Immune switch from a Th2 to a Th1 and Th17 response (Fonseca AB de et al. 2017 ).
In contrast to the hyperergic immunopathogenesis of T1R, T2R is r elativ el y aner gic despite being mediated by immune complexes (type III hypersensitivity) (Mitra 2017 ).Not surprisingly, T2R mor e commonl y occurs in the anergic spectrums of leprosy (i.e.lepromatous and borderline lepromatous types of leprosy).These clinical types are associated with high bacillary indices ranging from 4 + to 6 + on the logarithmic Ridley scale out of a maximum of 6 + .Patients with a high bacterial load of M. leprae or M. lepromatosis are more likely to have T2R and not T1R (Nunzi et al. 2020 ).
Mor eov er, these c hanges in the host imm une r esponse can be induced b y infections, pregnanc y, and other factors associated with the host, which could induce a humor al imm une r esponse and the further release of antibodies that may interact with mycobacterial antigens resulting in immune complexes (Galeano et al. 2023 ).Our hypothesis is that these immune complexes could r ecruit basophils, whic h would degr anulate and r elease v asoactive amines promoting vasodilation and increasing vascular endothelial permeability, thus facilitating the deposition of immune complexes in different tissues.
Once immune complexes have been deposited within the tissue, the y can acti vate the classical complement pathway and T3R or Lucio's phenomenon could be regarded as a variant of T2R known as necrotizing erythema nodosum (NEN) (Ranugha et al. 2013, Pinheiro et al. 2022 ).
The T3R could r epr esent a type III hypersensitivity e v ent because patients with DLL have a high antigenic load (6 + ) (Cruz et al. 2023 ).The ele v ated antigenic burden may precipitate the generation of immune complexes, thus promoting activation of the classical complement pathway, as pr e viousl y noted.Howe v er, this hypothesis r equir es further v alidation (Jur ado et al. 2015, Sharma et al. 2019 ).The accum ulation of imm une complexes in medium and small-sized vessels (Cruz et al. 2023 ) triggers phagocyte migr ation, r esulting in high secretion levels of proinflammatory cytokines (TNF-α and IFN-γ ) (Misra et al. 2014, Polycarpou et al. 2017 ).Additionall y, thr ombus formation due to platelet a ggr egation in medium and small-sized vessels induces a v asculonecr otic r eaction, clinicall y pr esenting as m ultiple, potentiall y lar ge and extensive skin ulcers (Tajalli and Wambier 2021 ).
Accum ulation of imm une complexes in the medium and small-sized vessels (Misra et al. 2014 ) leads to phagocyte migration, ther efor e high secr etion le v els of pr oinflammatory cytokines (TNF-α e IFN-γ ) than the T2R in a classic erythema nodosum lepr osum (ENL).Thr ombus formation due to platelet a ggr egation in medium and small-size vessels induces a vasculonecrotic reaction that would manifest clinically as multiple skin ulcers, which This figur e shows that the formation of imm une complexes activ ates the classical complement pathway, leading to the recruitment of phagocytes that induce damage in various tissues (nerves, skin, kidneys, eyes, and testicles).can be quite large and extensive (Sharma et al. 2019 ). Figure 3 describes T3R-mediated skin tissue damage.

Clinical manifestations of leprosy reactions and their immunologic explanation
The T1R or RR is an acute inflammatory episode causing sudden worsening of existing skin lesions and nerve pain due to heightened imm une r esponse (Stefani et al. 2009 ).Besides, these pathological e v ents occur as indir ect consequences of these imm unological e v ents (Serr ano-Coll et al. 2018 ).The pro-inflammatory environment, fueled by TNF-α and IFN-γ , in conjunction with the LT CD8 Tc1 cytotoxic response, triggers cutaneous inflammation, Schwann cell (SC) damage, and demyelination (Serrano-Coll et al. 2018 ) (Table 1 ).The impairment of SCs disrupts the expected propagation of the electrical signal, typically conducted through the nodes of Ranvier via action potentials, leading to compromised depolarization and a subsequent impact on axonal conduction velocity (Alizadeh et al. 2015 ).T herefore , the demyelinated axons within a SC result in depolarization and a reduction in axonal conduction velocity (Alizadeh et al. 2015 ).T herefore , the severity of T1R could lead to accelerated demyelination of nerves, potentiall y r esulting in an acceler ated pr ogr ession to gr ade 2 disability in leprosy patients, characterized by impairments in the eyes, hands, and feet (Scollard 2008, Serrano-Coll et al. 2018 ). Figure 4 A-B, these figures show skin inflammation in T1R.
The primary manifestations in T2R encompass panniculitis and m ulti-or gan dysfunction (Hafsi and Badri 2023 ).Panniculitis is c har acterized by subcutaneous adipose tissue inflammation (panniculus adiposus), pr esenting clinicall y as subcutaneous nodules symmetrically distributed on the face, back, and upper limbs (Vashisht et al. 2022 ).Negera et al. ( 2017 ) demonstrated that neutr ophilic infiltr ation was observ ed in 58.8% of the samples with T2R, along with the presence of eosinophils, mast cells, T cells, and foamy histiocytes throughout the dermis and subcutis, explains the dermal inflammation.(Figure 4 C shows nodules in the skin T2R).In cases of T3R or necrotic erythema nodosum, a severe complication of lepromatous leprosy characterized by necrotizing v asculitis, extensiv e skin lesions and tissue necr osis primaril y affect the lo w er limbs due to phagocyte infiltration and massive deposition of platelets on endothelial and vascular tissues, triggering skin ischemia that can progress to ulcerations and necrosis (Sehgal 2005 ) (Table 1 ) and Fig. 4 D-E, these figures show the mechanisms of the ulcer ativ e lesions in lo w er limbs (NEN vs T3R).Nevertheless, a clinically distinguishing feature between these two occurrences is that NEN is often linked to fe v er and constitutional symptoms, whereas T3R is not (Sharma et al. 2019 ).
Another pertinent discussion topic is why patients with T2R experience he patic, kidne y, testicular, and ocular injuries when lepr osy primaril y affects the skin and nerv es.To compr ehensiv el y address this question, it is essential to recognize that leprosy reactions are associated with an anergic response (Leon et al. 2015 ).As a result, mycobacterial dissemination in anergic patients affects the skin and nerves and other tissues such as bone marrow, liv er, spleen, l ymph nodes , lungs , kidneys , eyes , and testes (Gautam et al. 2021 ).The immune complexes formed through antigenantibody interactions trigger an inflammatory response and tissue injury (Goulart et al. 2022, Dewi et al. 2023 ).

Medical management of leprosy reactions and their immunological explanation
In treating of LRs, prednisolone is critical in controlling T1R (Safa et al. 2009 ).This medication inhibits the differentiation of CD4 TL into an effector Th1 and T h17 phenotypes .It increases the activity of regulatory T cells, promoting the control of the exacerbated  cellular immune response seen in these patients (Della Corte and Morgillo 2019 ).The immunomodulation induced by this drug results in a reduction of inflammation in the skin and peripheral nerves .T he recommended prednisolone dose for managing T1R is 1 mg/kg/day, but in adults, it is advisable to start with a dose r anging fr om 40-60 mg/day and gr aduall y ta per it down (Safa et al. 2009, Nunzi et al. 2020 ).The duration of this ther a py is a subject of debate; ho w e v er, we belie v e it should be maintained for one to three months due to the numerous adverse effects associated with long-term use of prednisolone (Nunzi et al. 2020 ).See Table 1 .
Besides, in patients with T1R, it is crucial to consider nerve decompression as an approach to reduce the disability associated with these immunological events, particularly when the nerves demonstr ate entr a pment within the tunnels of the extr emities (Nickerson and Nickerson 2010 ).Decompression of peripheral nerves leads to a reduction in inflammation, which, from an imm unological perspectiv e, would decr ease pha gocyte c hemotaxis and, consequently, the release of proinflammatory cytokines TNFα, IFN-γ , both by these cells and CD4 and CD8 TL (Sc homber g et al. 2012 ).
On the other hand, Thalidomide has become the drug of choice for treating of T2R.Although this drug was initially synthesized in Germany in the 50 s to address hyperemesis gravidarum, it was withdrawn from the market in the 60 s due to its association with congenital malformations, particularly phocomelia (Talhari et al. 2006 ).Ho w e v er, in the 21st century, it was described the imm unomodulatory pr operties of this drug and its use in diseases related to type III hypersensitivity reactions, such as T2R (Talhari et al. 2006 , Gómez andCastro 2016 ).
The immunomodulatory effect of thalidomide could be related to the inhibition of TNF-α activity production.The reduction of le v els of this cytokine decreases the recruitment of phago-cytes, such as macrophages and neutrophils (Upputuri et al. 2020 ).
Ther efor e, r educing the migr ation of these cells is related to two beneficial effects in patients with T2R: 1. Pr e v enting the inefficient phagocytosis of immune complexes deposited in host tissues, and 2, inhibiting the release of proinflammatory cytokines (TNF-α, IFN-γ ) that are responsible for tissue damage.On the other hand, this medication can reduce the effector activity of CD4 and CD8 TL (Kim et al. 2015 ).Besides, thalidomide promotes the expression of the transcription factor FoxP3 + in specific subpopulations of CD4 TL, which is crucial in the conversion of these cells into iTr eg, whic h play a vital role in reducing inflammation by suppressing effector patterns such as Th1 and Th17 (Kim et al. 2015 ).See Table 1 .
The dosage of this medication will vary depending on the clinical se v erity of T2R.In mild cases, defined as ≤ 10 painful nodular lesions with limited systemic manifestations, a recommended dose of thalidomide is 100 to 200 mg daily (Talhari et al. 2006, Upputuri et al. 2020 ).For erythema nodosum leprosum, Lucio phenomenon, and se v er e T2R cases c har acterized by the presence of > 20 painful nodular lesions along with significant systemic manifestations, the thalidomide dosage ranges from 300 to 400 mg per day, with the suggested incor por ation of pr ednisolone at the pr e viousl y mentioned doses (Talhari et al. 2006, Upputuri et al. 2020 ).The duration of this medical approach will depend on each patient's clinical pr ogr ess, and it is al ways r ecommended to investigate the presence of an underlying infectious trigger for this imm unological e v ent (Galeano et al. 2023 ).Ne v ertheless, thalidomide should be avoided in women of childbearing age due to its potential teratogenic effects (Thangaraju et al. 2020 ).Ho w ever, it's worth noting that the use of thalidomide in the Lucio phenomenon (T3R) can be a subject of contr ov ersy among some authors, as they consider the utility of this medication may be limited and prefer to use Multibacillary-MDT and antibiotics for secondary infections (Rocha et al. 2016 ).The use of these antibiotics to treat T3R is supported by the fact that the extensive replication of these mycobacteria serves as the trigger for these reactions (Peixoto et al. 2013 ).Ther efor e, the administr ation of these drugs reduces mycobacterial replication and enhances the clinical outcomes in these patients (Peixoto et al. 2013 ).See Table 1 .

Conclusions and perspective
Lepr osy r eactions ar e complex imm une-r elated e v ents that can result in disability or e v en death.Ther efor e, enhancing our understanding of lepr osy r eactions could lead to the discovery of ne w and v aluable biomarkers and imm unomodulators for dia gnosing and treating of lepr osy.In tr opical r egions, lepr osy r eactions are often underdiagnosed as they are not typically considered in the differential diagnosis for febrile syndromes, which can hinder medical attention for leprosy patients.Consequently, educating healthcar e pr ofessionals about these hypersensitivity r eactions is crucial since they affect more than half of all leprosy patients.
Ho w e v er, se v er al questions remain unanswered: What are the tr anscriptomic c hanges associated with LRs?What other molecular and imm une mec hanisms might be linked to LRs? Ultimately, finding answers to these questions is essential for enhancing the quality of life for leprosy patients and pr e v enting complications arising from these immune events.

Figure 1 .
Figure1.Imm une mec hanism of tissue dama ge during T1R.This figur e shows an incr ease in the activity of CD4 T cells with T h1 and T h17 patterns and CD8 Tc1 cells, which induce a proinflammatory environment, resulting in collateral damage to tissues affected by M. leprae (skin and peripheral nerves).

Figure 2 .
Figure 2. Imm une mec hanism of tissue dama ge during T2R.This figur e shows that the formation of imm une complexes activ ates the classical complement pathway, leading to the recruitment of phagocytes that induce damage in various tissues (nerves, skin, kidneys, eyes, and testicles).

Figure 3 .
Figure 3. Imm une mec hanism of ulcer ation during T3R.This figur e shows that the formation of imm une complexes can activ ate the classical complement pathway, leading to the recruitment of predominantly polymorphonuclear phagocytes, along with significant platelet aggregation in blood vessels, forming thrombus and inducing necrotic vasculopathy.

Figure 4
Figure 4 A-E.Skin manifestations of leprosy reactions.A-B.These images show inflammation and erythema in the skin lesions.C.This image shows subcutaneous nodules in a patient with T2R.D-E.These figures show ulcerative lesions in lo w er limbs (NEN vs T3R).NEN: Necrotizing erythema nodosum, T3R: Type 3 leprosy reactions.